Image sensor readout method and apparatus

ABSTRACT

A pixel readout circuit including at least first, second and third memory locations. During an integration period of a pixel, the pixel readout circuit repeatedly samples the pixel output level during the integration period, stores the first sample in the first memory location, and stores each subsequent sample in memory locations other than the first memory location. Each sample is stored with a time corresponding to when that sample was taken, such that at any one time subsequent to the first three samples having been stored, at least the first sample and the two most recent samples are stored. Also disclosed is a corresponding method of reading out of a pixel output over an undefined integration period.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of Great Britain patentapplication number 1121575.3, filed on Dec. 15, 2011, which is herebyincorporated by reference to the maximum extent allowable by law.

BACKGROUND

1. Technical Field

The present disclosure relates to the field of digital image sensors,and in particular to the field of high dynamic range methods for suchsensors.

2. Discussion of the Related Art

Digital image sensing based upon solid state technology is well known,the two most common types of image sensors currently being chargecoupled devices (CCDs) and complementary metal oxide semiconductor(CMOS) image sensors. Digital image sensors are incorporated within awide variety of devices throughout the consumer, industrial and defencesectors among others.

An image sensor is a device comprising one or more radiation sensitiveelements having an electrical property that changes when radiation isincident upon them, together with circuitry for converting the changedelectrical property into a signal. As an example, an image sensor maycomprise a photodetector that generates a charge when radiation isincident upon it. The photodetector may be designed to be sensitive toelectromagnetic radiation in the range of (human) visible wavelengths,or other neighbouring wavelength ranges, such as infra red or ultraviolet for example. Circuitry is provided that collects and carries thecharge from the radiation sensitive element for conversion to a valuerepresenting the intensity of incident radiation.

Typically, more than one radiation sensitive element will be provided inan array. The term pixel is used as a shorthand for picture element. Inthe context of a digital image sensor, a pixel refers to that portion ofthe image sensor that contributes one value representative of theradiation intensity at that point on the array. These pixel values arecombined to reproduce a scene that is to be imaged by the sensor. Aplurality of pixel values can be referred to collectively as image data.Pixels are usually formed on and/or within a semiconductor substrate. Infact, the radiation sensitive element comprises only a part of thepixel, and only part of the pixel's surface area (the proportion of thepixel area that the radiation sensitive element takes up is known as thefill factor). Other parts of the pixel are taken up by metalization suchas transistor gates and so on. Other image sensor components, such asreadout electronics, analog to digital conversion circuitry and so onmay be provided at least partially as part of each pixel, depending onthe pixel architecture.

One of the most important characteristics of any image sensor is itsdynamic range, that is, the ratio between the minimum and the maximumsignal that can be successfully reproduced by the image sensor. Thereare various fields in which a high or very high dynamic range isrequired.

One such device where a wide dynamic range is required is a biosensor.In a biosensor, each pixel is exposed to a substance suspected ofcontaining target diseases/chemicals/proteins etc. which is then treatedwith a chemical that reacts specifically with the target. This reactionproduces light which is then detected by the biosensor. Differenttargets and their corresponding chemicals can be put on different pixelson the biosensor array, such that the system can analyze a wide range ofsamples at once. However, it is often the situation that each pixel willreceive vastly different light levels. Some pixels saturate in fractionsof seconds, while others may take up to 30 seconds to saturate.

It would be desirable, therefore, to provide readout circuitry able toanalyze all these signals, therefore greatly increasing the pixelarray's dynamic range.

SUMMARY

In a first aspect there is provided a pixel readout circuit comprisingat least first, second and third memory locations wherein, during anintegration period of a pixel, said pixel readout circuit is operable torepeatedly sample the pixel output level during the integration period,to store the first sample in said first memory location, and to storeeach subsequent sample in memory locations other than said first memorylocation, each sample being stored with a time corresponding to whenthat sample was taken, such that at any one time subsequent to the firstthree samples having been stored, at least the first sample and the twomost recent samples are stored.

Said memory locations may comprise separate SRAMs. Three SRAMs may beprovided per readout circuit. Each sample subsequent to the first andits corresponding time may be alternately stored in said second andthird memory locations.

Said pixel readout circuit may be operable to sample the pixel outputlevel at set intervals.

The pixel readout circuit may be operable to cease sampling of the pixeloutput level when the pixel output level reaches a predeterminedthreshold, which may be determined relative to the pixel saturationlevel. The pixel readout circuit may be operable to perform correlatedsampling of said pixel output, using the contents of said first memorylocation and the contents of the memory location in which thepenultimate sample taken is stored.

The pixel readout circuit may comprise a global counter for calculatingsaid times corresponding to when each sample was taken, relative to thebeginning of the pixel integration period.

The pixel readout circuit may comprise a ramp generator that may beoperable to begin ramping at the beginning of each sample.

Said pixel readout circuit is further operable to store the time takento pixel saturation.

In a further aspect there is provided a digital image sensor comprisingan array of pixels, wherein each pixel has a corresponding pixel readoutcircuit according to the first aspect of the invention. Also provided isa biosensor comprising such a digital image sensor.

In a yet further aspect there may be provided of reading out of a pixeloutput over an undefined integration period, comprising the steps of:repeatedly sampling the pixel output level during said integrationperiod; storing the first sample in a first memory location along with atime corresponding to when the first sample was taken; storing eachsubsequent sample in memory locations other than said first memorylocation along with a time corresponding to when that sample was taken,such that at any one time at least the first sample and the two mostrecent samples are stored.

Said subsequent samples may be taken and stored until such time as thepixel output level reaches a predetermined threshold. Said predeterminedthreshold may be determined relative to the pixel saturation level. Oncesaid sampling has ceased, the contents of said first memory location andthe contents of the memory location in which the penultimate sampletaken is stored may be used in correlated sampling of the pixel output.

The method may be performed for multiple pixels over differentintegration periods. Said method may further include storing the timetaken to saturation for the/each pixel.

Said method may further include sampling the pixel output level at setintervals.

Said method may comprise a method of performing correlated doublesampling. If so, each subsequent sample, and its corresponding time, maybe alternately stored in second and third memory locations.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the disclosure will now be described, by way of example only,with reference to the accompanying drawings in which:

FIG. 1 is a block diagram of one of the operating mechanisms of afluorescence biosensor according to the prior art;

FIG. 2 is a block diagram of a generic conventional fluorescencebiosensor according to the prior art;

FIG. 3 is a pixel readout circuit according to an embodiment;

FIG. 4 is a timing diagram showing a operation embodiment of the pixelreadout circuit of FIG. 3; and

FIG. 5 is a diagram illustrating how the memory contents of the of thepixel readout circuit of FIG. 3 change over time.

DETAILED DESCRIPTION

Biosensors are analytical tools that detect the presence of a chemicalor biochemical species in a complex mixture by combining the molecularrecognition properties of biological macromolecules (e.g., enzymes,antibodies, DNA or even whole cells) with signal transduction mechanisms(e.g., optical or electrochemical) that couple ligand bindings withreadily detectable physical changes.

The optical signal transduction mechanisms employed by biosensors arebased on luminescence spectroscopy, absorption spectroscopy (ultraviolet(UV) to deep infrared (IR)), Raman or fluorescence spectroscopy.Fluorescence spectroscopy will be described in detail below, purely byway of example. However the readout circuitry and photodetectorarrangements disclosed herein are equally applicable to any other typeof optical biosensor such as those mentioned above, as well as to otherphotodetector/imaging applications in general.

To increase analytical throughput, an analytical process should becapable of simultaneously detecting a number of different species.Biosensor arrays achieve this by assembling a large number of differentbiological macromolecules (each of which contains a recognition site fora given biological species, with such species being known henceforth asan analyte) into densely packed arrays of unique sensor elements.

Biosensor arrays have three main operational mechanisms, namely labeledanalyte pooling (used in DNA and RNA hybridization assays), sandwichassays (used for antibody recognition) and direct assays.

Referring to FIG. 1, during labeled analyte pooling a biologicalmacromolecule 10 specific for an analyte 12 is immobilized on a solidsupport 14. A sample (containing an analyte 12) is mixed with a solutionof a fluorescent label 16 that binds to the analyte 12 therein. Thesample is then introduced to the biosensor array and the analyte 12therein is bound to the biological macromolecule 10 specific therefore.Other species 18, 20 in the sample that are not of analytical interest(and for which there are no biological macromolecules immobilized on thesolid support 14) remain in free solution.

The support 14 is then washed with a cleaning solution (not shown) andany unbound species in the sample are flushed therefrom, leaving thefluorescently labeled analyte 12 bound to the support 14. Thefluorescently labeled analyte 12 fluoresces when exposed to radiation(e.g., from an IR laser) and the resulting fluorescent pattern of thebiosensor array acts as a biochemical fingerprint that can be readilyimaged.

Referring to FIG. 2, a fluorescence biosensor 30 typically comprises astimulating light source 32, a substrate 34 and a photodetector 36. Thesubstrate 34 comprises a plurality of sensor elements 37 a, 37 b and 37c each of which comprises an immobilized biological macromolecule 38 a,38 b and 38 c specific for a particular analyte of interest. While FIG.2 shows the photodetector 36 disposed remotely from the substrate 34,nonetheless, it will be appreciated that this arrangement is notessential and the substrate 34 could alternatively be configured tohouse both the photodetector 36 and the sensor elements 37 a, 37 b and37 c.

Using, for example, the above-described labeled analyte pooling scheme,analytes 40 a and 40 c in a sample have fluorescent labels boundthereto. When the sample (not shown) is introduced to the substrate 34,the labeled analytes 40 a and 40 c bind to the appropriate macromolecule38 a and 38 c. However, if an analyte that binds to a particularmacromolecule 38 b is not present in the sample, the correspondingsensor element 37 b remains free of labeling.

The light source 32 emits light 42 of wavelength μ₁, which is astimulating wavelength for the fluorescent labels (bound to the analytes40 a and 40 c). The light source 32 is positioned so that the light 42it emits falls upon the sensor elements 37 a, 37 b and 37 c (and anyfluorescently labeled analytes bound thereto). It will be appreciatedthat there may be some additional optical elements (e.g., lens,lightguide, etc.) disposed between the light source 32 and the sensorelements 37 a, 37 b and 37 c. It will also be appreciated that the lightsource 32 may alternatively scan the array of sensor elements 37 a, 37 band 37 c. The light 42 stimulates the fluorescent labels bound to theanalytes 40 a and 40 c to emit radiation of wavelength λ₂ (λ₁<λ₂)

The photodetector 36 comprises a plurality of pixels 44 a, 44 b and 44c, each of which is positioned to detect the radiation emitted from agiven sensor element 37 a, 37 b and 37 c. As before, it will beappreciated that there may be some additional optical elements (e.g.lens, a light guide, etc.) disposed between the sensor elements 37 a, 37b and 37 c and the photodetector 36. It will also be appreciated thatthe photodetector 36 may alternatively scan the array of sensor elements37 a, 37 b and 37 c.

Radiation may be emitted from sensor elements 37 a, 37 b and 37 c atvastly differing rates such that, over a given time, different pixels 44a, 44 b and 44 c may receive vastly differing light levels.Consequently, some pixels saturate in fractions of seconds, while othersmay take up to 30 seconds to saturate. Therefore the individual pixelsof the pixel array may be exposed with varying integration times. Thisprovides a challenge for the readout circuitry for the pixel array.

It is proposed to provide a readout circuit which samples the voltagelevel of a pixel output repeatedly at set intervals during the pixelintegration time.

FIG. 3 shows a readout circuit 300 according to an embodiment. It isproposed that one such readout circuit 300 is provided for each pixel ofthe pixel array. The readout circuit 300 comprises a comparator 310, theoutput of which is connected to three (in this example) static randomaccess memories SRAMs 330 a, 330 b, 330 c via a multiplexer 320. Thecomparator 320 receives the pixel output signal VPIX and a ramp signalVRAMP generated by a digital to analogue converter (DAC) 340. The DACalso provides a global counter output signal DACCOUNT which is used toincrement a global counter 350. This is separate from the analogue todigital converter (ADC) and its associated counter (not shown), whichoperates in a known way to provide a digital equivalent to the signalVPIX each time that it is sampled.

This circuit is operable to compare the signal VRAMP from the DAC 340 tothe pixel output signal VPIX. When the signals VPIX and VRAMP cross, theoutput COMPOUT of comparator 310 will flip and a digital representationof pixel output VPIX (as converted by an ADC, not shown) will be storedon one of the SRAMs 330 a, 330 b, 330 c. The appropriate SRAM 330 a, 330b, 330 c is selected using the input SRAMSEL of multiplexer 320. Thevalues stored in the SRAMs 330 a, 330 b, 330 c can be read out asSRAM1OUT, SRAM2OUT and SRAM3OUT when required. Each SRAM 330 a, 330 b,330 c also receives counter signal GCC (Gray Code Count).

FIG. 4 is a timing diagram showing the Pixel reset signal RST, timersignal DACCOUNT, pixel output signal VPIX and ramp signal VRAMP overtime during an operational embodiment of the readout circuit of FIG. 3.FIG. 5 illustrates how the contents of the SRAMs 330 a, 330 b, 330 cchange over time.

In this operational embodiment, the pixel output signal VPIX is sampledimmediately after pixel reset. A digital representation of pixel outputVPIX₁ after a configurable time At is stored in one of the SRAMs, forexample SRAM1 330 a. Also stored is the time t₁, as counted by the DACglobal counter (signal DACCOUNT), at which this initial reading wastaken. These values remain stored during the entire integration periodof the pixel, and can be used as the “black level” or base reading in acorrelated double sampling (CDS) calculation (or correlated multiplesampling calculation where there are more than three SRAMs), incombination with the final stored pixel output level.

Subsequent to this initial value being taken, the pixel output levelVPIX is repeatedly sampled at set sampling points separated by time At,during the pixel integration period. The (digital) pixel output levelVPIX sampled at each time is stored in an alternate one of SRAM2 330 band SRAM3 330 c, overwriting any prior stored value in the process. Alsostored with each pixel output value VPIX is the corresponding time thatthe level was sampled, as counted by the DAC's global counter DACCOUNT.This time is used to place each of these pixel output values VPIX atspecific times along the decay time of the VPIX signal. This allows CDSsampling as voltages with specific sampling times along the decay linecan be taken.

In the specific example shown in FIGS. 4 and 5, SRAM2 330 b stores thepixel output value VPIX₂ and the time it was sampled t₂. Subsequently,SRAM3 330 c stores the pixel output value VPIX₃ and time t₃, after whichSRAM2 330 b stores the pixel output value VPIX₄ and time t₄ (overwritingVPIX₂ and t₂ in the process). This is continued until the pixel issaturated (VPIX=VPIX_(MAX)), or pixel output signal VPIX reaches anotherpredetermined threshold (for example VPIX=0.9VPIX_(MAX)). When it isdetermined that the last stored value is the maximum saturated value ofthe pixel output VPIX, sampling is immediately stopped, and thepenultimate sample value and corresponding time is used as the finalpixel reading in the CDS calculation, in combination with the blacklevel values stored initially. In the specific example shown in FIG. 5,the values used are those ringed with a dashed line, that is VPIX₁ andt₁ stored on SRAM1 330 a and VPIX₄ and t₄ stored on SRAM2 330 b. Thetime to saturation t₅ can also be stored, providing more information onthe specific pixel.

A pixel array with varying integration times for each pixel can utilisethis circuitry to read out any pixel, regardless of the arrayintegration time or pixel voltage gradient using CDS under a single rampADC. Such circuitry can operate on systems without auto exposurecontrol, and allows the pixel voltage to ramp at a wide range ofgradients, be it as a result of a charge integrator or any other slopedependent readout.

Various improvements and modifications may be made to the above withoutdeparting from the scope of the disclosure. For example, more than threeSRAMs may be used, particularly where correlated multiple sampling isdesirable. And other types of memory or storage could be used in placeof SRAM. Also, while the readout circuit is described in relation to usewith a biosensor, it may be used with any pixel array, specificallywhere a large dynamic range is desirable. This may include imagingdevices such as inter alia a camera.

Such alterations, modifications, and improvements are intended to bewithin the spirit and scope of the invention. Accordingly, the foregoingdescription is by way of example only and is not intended as limiting.The invention is limited only as defined in the following claims and theequivalents thereto.

What is claimed is:
 1. A pixel readout circuit comprising at leastfirst, second and third memory locations wherein, during an integrationperiod of a pixel, said pixel readout circuit is operable to repeatedlysample the pixel output level during the integration period, to storethe first sample in said first memory location, and to store eachsubsequent sample in memory locations other than said first memorylocation, each sample being stored with a time corresponding to whenthat sample was taken, such that at any one time subsequent to the firstthree samples having been stored, at least the first sample and the twomost recent samples are stored.
 2. A pixel readout circuit as claimed inclaim 1 wherein said memory locations comprise separate SRAMs.
 3. Apixel readout circuit as claimed in claim 1, wherein each samplesubsequent to the first and its corresponding time is alternately storedin said second and third memory locations.
 4. A pixel readout circuit asclaimed in claim 1, wherein said pixel readout circuit is operable tosample the pixel output level at set intervals.
 5. A pixel readoutcircuit as claimed in claim 1, operable to cease sampling of the pixeloutput level when the pixel output level reaches a predeterminedthreshold.
 6. A pixel readout circuit as claimed in claim 5 operablesuch that said predetermined threshold is determined relative to thepixel saturation level.
 7. A pixel readout circuit as claimed in claim 5operable, once said sampling has ceased, to perform correlated samplingof said pixel output using the contents of said first memory locationand the contents of the memory location in which the penultimate sampletaken is stored.
 8. A pixel readout circuit as claimed in claim 1,wherein said pixel readout circuit comprises a global counter operableto calculate said times corresponding to when each sample was taken,relative to the beginning of the pixel integration period.
 9. A pixelreadout circuit as claimed in claim 1, wherein said pixel readoutcircuit comprises a ramp generator that is operable to begin ramping atthe beginning of each sample.
 10. A pixel readout circuit as claimed inclaim 1, wherein said pixel readout circuit is further operable to storethe time taken to pixel saturation.
 11. A digital image sensorcomprising an array of pixels, wherein each pixel has a correspondingpixel readout circuit according to claim
 1. 12. A biosensor comprising adigital image sensor according to claim
 9. 13. A method of reading outof a pixel output over an undefined integration period, comprising thesteps of: repeatedly sampling the pixel output level during saidintegration period; storing the first sample in a first memory locationalong with a time corresponding to when the first sample was taken;storing each subsequent sample in memory locations other than said firstmemory location along with a time corresponding to when that sample wastaken, such that at any one time at least the first sample and the twomost recent samples are stored.
 14. The method of claim 13 wherein saidsubsequent samples are taken and stored until such time as the pixeloutput level reaches a predetermined threshold.
 15. The method of claim14 wherein said predetermined threshold is determined relative to thepixel saturation level.
 16. The method of claim 14 further comprising,once said sampling has ceased, using the contents of said first memorylocation and the contents of the memory location in which thepenultimate sample taken is stored in correlated sampling of the pixeloutput.
 17. The method of claim 13, being performed for multiple pixelsover different integration periods.
 18. The method of claim 13, whereinsaid method further includes storing the time taken to saturation forthe/each pixel.
 19. The method of claim 13, further including samplingthe pixel output level at set intervals.
 20. The method of claim 13,wherein each subsequent sample, and its corresponding time, isalternately stored in second and third memory locations, said methodcomprising a method of performing correlated double sampling.